1. Field of the Invention
The present invention relates to apparatus for control of modes of oscillation in lasers.
2. Description of Related Art
Lasers are characterized by a resonant laser cavity defined by reflective surfaces. For the typical laser, two mirrors are used. In commercial lasers, a common cavity called near-hemispheric, includes a spherical mirror at one end and an optically flat mirror at the other. The radius of curvature of the spherical mirror is set so that the center of curvature of the spherical mirror lies beyond the flat mirror. This near-hemispheric arrangement avoids many of the problems of alignment suffered by other types of laser cavities. Further, this type of cavity serves as an excellent example for demonstrating the operation of the present invention.
A given laser, including laser medium and a resonant cavity, will generally support a plurality of modes of oscillation. These modes of oscillation will include different wavelengths, for instance, 4579 Angstroms and 5145 Angstroms, (among others) for the argon ion laser. Further, different transverse modes of oscillation clustering around the primary wavelengths, will occur within the same cavity. For many applications, only one transverse mode designated the TEM.sub.00, or the lowest order mode, is desired. The lowest order mode typically has a smaller and more uniform beam cross-section than higher order modes. The size of modes within the laser cavity, called mode size, is one characteristic of modes that is used to discriminate between desired and undesired modes. The cross-section of lower order modes within the laser cavity is smaller than the cross-section of higher order modes. Also, the cross-sections of the modes of longer wavelengths are greater than the cross-sections of corresponding modes in shorter wavelengths.
In the near-hemispheric example discussed above, the lowest order modes are essentially cone-shaped with the smaller end of the cone on the flat mirror. The cross-sectional size of a given mode at any point within the laser cavity is determined by the radius of curvature of the spherical mirror.
Suppression of transverse modes in the prior art has been accomplished using apertures within the laser cavity which block oscillation of modes having a larger cross-sectional size than the aperture at the location within the cavity at which the aperture is placed. Alternatively, the bore size of a gas laser can be selected so that it forms an effective aperture suppressing unwanted modes of oscillation.
Problems arise in lasers employing apertures if the user desires to change the frequency of light generated by the laser. The oscillation can be started by adjusting prisms or other filtering components in the laser to filter out unwanted wavelengths and allow oscillation of the selected wavelength. However, the mode size changes when the wavelength is changed. Therefore, the aperture or bore size will no longer be optimal and may allow oscillation of unwanted transverse modes. Alternatively, the aperture or bore size may be too small and suppress the desired wavelength. Prior art systems have addressed the problem by providing adjustable apertures with lasers. However, adjustable apertures are complicated and expensive, and are particularly difficult to install in sealed gas lasers.
In lasers which rely on the bore size of the laser to suppress unwanted modes of oscillation, an additional problem arises due to erosion of the bore. Inevitably, laser bores will erode due to natural sputtering action over the life of the laser. As the bore erodes, the effective aperture size increases. As the aperture size increases, transverse modes of oscillation will begin to occur. These transverse modes will compete with the available population inversion in the laser medium and decrease the usable TEM.sub.00 output power of the laser. Further, the quality of the beam output will be reduced.